An absorbent article such as a disposable diaper, a sanitary napkin, and an incontinence pad includes an absorber that absorbs and retains body fluid excreted from a body such as urine and menstrual blood, a flexible liquid-permeable top sheet disposed on a body-contacting side, and a liquid-non-permeable back sheet disposed on a side opposite to the body-contacting side. The absorber is generally composed of: a hydrophilic fibrous base material such as wood pulp; and a water-absorbent resin powder. Body fluid passes through the top sheet made of a nonwoven fabric or the like and is absorbed by the absorber. The absorbed body fluid is diffused by the fibrous base material such as wood pulp within the absorber, and absorbed and retained by the water-absorbent resin powder.
Patent Literatures 1 to 4 propose water-absorbent resin powders that can be used in absorbers. Patent Literature 1 discloses an absorbent resin particle that contains a crosslinked polymer (A1) containing a water-soluble vinyl monomer (a1) and/or a hydrolyzable vinyl monomer (a2) and a crosslinking agent (b) as essential constitutional units and in which a hydrophobic material (C) is present within the absorbent resin particle in an amount of 0.01 to 10.0% by weight with respect to the weight of the crosslinked polymer (A1) and a hydrophobic material (D) is present on the surface of the absorbent resin particle in an amount of 0.001 to 1.0% by weight with respect to the weight of the crosslinked polymer (A1).
Patent Literature 2 discloses a method for manufacturing a water-absorbent resin particle (D), the method including: a first step of conducting reversed-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer solution containing a crosslinking agent, in a hydrophobic organic solvent in the presence of a dispersant to obtain a reaction mixture (A) containing a water-containing water-absorbent resin particle (a); a second step of further adding a water-soluble ethylenically unsaturated monomer solution containing a crosslinking agent to the (A) and conducting reversed-phase suspension polymerization to obtain a reaction mixture (B) containing a water-containing water-absorbent resin particle (b); a third step of dehydrating and desolvating the (B) to obtain a water-absorbent resin cake (C); and a fourth step of drying the water-absorbent resin cake, wherein the bulk density of the water-containing water-absorbent resin particle (b) is 0.25 to 0.35 g/ml and the bulk density of the water-absorbent resin particle (D) after drying is 0.45 to 0.55 g/ml.
Patent Literature 3 discloses an absorbent article that contains: a water-absorbent resin (i) whose gel liquid-passing rate (ml/min) is from 0.01 to 3; and a water-absorbent resin (ii) whose gel liquid-passing rate (ml/min) is from 5 to 200.
Patent Literature 4 discloses a water-absorbent resin composition characterized by containing a water-absorbent resin (A) and a modifier (B) whose surface tension is 10 to 30 dyne/cm and which has a binding group that can chemically bind to the water-absorbent resin (A).
Meanwhile, in order to make an absorbent article thin, reduction of the amount of the fibrous base material and increase of the amount of the water-absorbent resin powder in the absorber have been addressed. Such a thin absorber has a problem of so-called gel blocking. In other words, when an absorber in which the amount of the fibrous base material is reduced and the amount of the water-absorbent resin powder is increased absorbs body fluid, the water-absorbent resin powder on the skin-contacting side initially absorbs the body fluid and swells. However, since the content of the water-absorbent resin powder in the absorber is high, the swollen water-absorbent resin powders are likely to come into contact with each other. As a result, a void as a passage for body fluid is closed, and the absorber cannot exert a certain absorbing ability. This phenomenon is called gel blocking. The technologies for improving the gel blocking phenomenon are disclosed in Patent Literature 5 to 8.
Patent Literature 5 discloses an absorbent article that includes a liquid-permeable top sheet, a liquid-non-permeable back sheet, and a liquid-retentive absorber interposed between both sheets and is characterized in that the absorber meets the following conditions 1) and 1′) and a high-water-absorbent polymer contained in the absorber meets the following conditions 2) and 3).
1) The absorber contains a fibrous base material and the high-water-absorbent polymer as principal components and 45 to 90% by weight of the total weight of the absorber is the high-water-absorbent polymer.
1′) The absorber is composed of two or more layers, at least one of the layers is a layer made of the fibrous base material, at least of the other layers is a layer made of the high-water-absorbent polymer or a layer made of a mixture of the high-water-absorbent polymer and the fibrous base material, and the layer made of the fibrous base material is a sheet-shaped layer obtained by adhering fiberized and laminated pulp fibers by a binder.
2) The high-water-absorbent polymer is made of a crosslinked body particle having a high crosslinking density in a surface portion thereof, and its absorption amount of a saline by a centrifugal dehydration method is equal to or greater than 25 g/g.
3) When: a cylinder having a cross-sectional area of 4.91 cm2 (an inner diameter: 25 mm phi) is filled with 0.5 g of the high-water-absorbent polymer and a saline; the high-water-absorbent polymer is swollen by the saline until reaching a saturated state; and 50 ml of the saline is passed through the cylinder after the swollen high-water-absorbent polymer sediments, a liquid-passing time is equal to or less than 20 seconds.
Patent Literature 6 discloses an absorbent article that includes a top sheet, a back sheet, and an absorber interposed between both sheets and containing a water-absorbent polymer and a fiber, and in which the absorber contains a water-absorbent polymer whose swollen gel has a repose angle equal to or less than 45 degrees, as the water-absorbent polymer, and has a water-absorbent polymer high concentration region where the content of the water-absorbent polymer exceeds a water-absorbent polymer average content calculated by the following equation.Water-absorbent polymer average content (% by mass)=(total mass of all water-absorbent polymer contained in absorber/total mass of absorber)×100
Patent Literature 7 discloses a body fluid absorbent article that includes a back sheet, a liquid-permeable top sheet, and an absorber interposed between both sheets and composed of a pulp and a high-absorbent polymer and in which the ratio of the high-absorbent polymer to the total weight of the pulp and the high-absorbent polymer of the absorber is 30 to 60% by weight, and with regard to the high-absorbent polymer, in its polymer particle size distribution, particles with a size of 500 micrometers or greater are 10% by weight, particles with a size of 250 to 500 micrometers are 70% by weight, and particles with a size of 250 micrometers or less are 20% by weight, the degree of ununiformity of its polymer particle shape is 0.3 to 0.5 g/ml in bulk density evaluation, and its body fluid sucking/absorbing performance of shifting body fluid retained in a pulp fiber void toward the high-absorbent polymer side is equal to or greater than 5 g/g for 15 seconds from start of absorption.
Patent literature 8 discloses a disposable absorbent article comprising an absorbent mat between a liquid-permeable top sheet and a liquid-impermeable back sheet, the absorbent mat comprising, in the order recited from the top sheet side, a sheet-shaped water-absorbent layer that contains a water-absorbent resin powder but that does not contain pulp fibers; and a fiber assembly layer that contains a water-absorbent resin powder and pulp fibers; wherein the sheet-shaped water-absorbent layer includes a plurality of water-absorbent resin powder presence regions in each of which the water-absorbent resin powder is wrapped, a plurality of water-absorbent resin powder absence regions each being formed between the two adjacent water-absorbent resin powder presence regions.